In marine applications wherein a gearbox is used to change the direction of rotation for propeller shafts, actuators are generally used to change the gear drive arrangement. In most cases gearboxes have three positions, forward propulsion, reverse propulsion and neutral.
Actuators presently used for changing gear drives in marine applications may be hydraulic or pneumatic operated. Provisions are made in certain instances for returning the gear change lever to the neutral position if there is a sudden loss of hydraulic or pneumatic pressure. This avoids situations where a power breakdown leaves the propulsion engaged.
Up until the present time there has been no electrically operated actuator which has a self centering arrangement so that if there is a power failure the actuator returns a gear change lever to the neutral position. There is definitely a requirement for such an actuator control for safety reasons and to permit the use of electrical power for controlling a ship thus avoiding the necessity of having to have either pneumatic or hydraulic controls.